In sound-masking applications, sound-masking systems are used to enhance speech privacy and comfort of workers in a working environment, for example. The principal of sound-masking is to increase the background noise of a room just enough to mask any distracting noise. The distracting noise generally comprises short acoustic events containing information, such as for example conversation, printer noise, telephone ring, etc. . . .
As sound-masking systems are being developed, it is now established that their efficiency is linked to their ability to emit an ideal masking sound spectrum with an adequate precision. The ideal masking sound is defined as achieving an optimized speech privacy at a listener's position for example (Acoustical Design of Conventional Open Plan Offices, Institute, for research I Construction, National Research Council, Canadian Acoustic, vol. 27. No. 3 (2003)-23).
Two parameters are mostly considered to obtain optimal privacy and comfort of workers, for example, with a sound-masking system: 1) the spectral shape of the masking sound and 2) the global level (or volume) of the masking sound.
To obtain the ideal spectral shape of the masking sound, the equalizer of the masking system is adjusted for each environment, taking into account a number of parameters including the size of the room, any coating on the walls of the room and the furniture in the room for example. The adjustment of the equalizer can be done manually. Automatic calibration systems are also known, as described for example in patent application US2006/0009969 A1 entitled “Auto-adjustment sound-masking system and method”.
One the one hand, for an optimum efficiency, the global level, or volume, of the masking sound is also be set according to the dedication of the room: for instance, the level of sound-masking in the hall of a bank will typically be set to a higher level then the sound-masking level in the open office of clerical workers, while the sound-masking level in a closed office will be set lower.
On the other hand, for an optimum comfort, the sound-masking level is adjusted according to the intensity of the activity in the zone to be monitored: when the environment becomes quite, such as during outside office hours for instance, high level of masking sound is not necessary and, on the contrary, the sound-masking level may need to be reduced to provide an optimum comfort to any workers still present.
To meet such time variations, some sound-masking systems currently available on the market include a volume calendar that allows specifying the global level of the masking sound over time. This feature allows setting a lower masking sound outside office hours and a higher masking sound during periods of the day when noisy activities are expected for example. However, as such systems do not include retroaction on the real acoustical activity, a wrong global level of the masking sound related to the intensity of the activity in the room often results.
An alternative to a volume calendar is a dynamic controller using sensors located in the room for picking up the ambient noise and increasing the sound-masking levels when the ambient noise due to current activity increases. The input of the controller is the signal coming from microphones located in the room where the masking sound volume must be controlled. The global level measured at the microphones is thus used to obtain an acoustic activity rating and to determine the needed masking sound volume over time.
However, using a retroaction based on the global energy to adjust the masking sound volume can suffer from instability, since, if the controller increases the masking sound volume in response to the increase in the ambient noise, then the global levels measured by the controller's microphones increase and, based on these new higher global levels, the controller continues to increase the masking sound level.
In case of paging applications, the global energy control system is stable since the sound signal is non-steady, and for these applications, the volume adjustment is generally based on the background noise measured in between calls. Sound-masking systems generate a constant signal and the noise measurements always take into account the masking sound. Increasing the sound-masking signal results in a steady increase of the global energy in the room, making a global energy controller unstable or at least imprecise.
In summary, sound-masking levels must be adjusted with a great precision to be efficient while not distressing. For example, in an open office, sound-masking typically varies from 43-45 dBA (unit relating to the use of a frequency weighting to approximate the human ear's response to sound) during the quiet time of the day to a maximum of 48 dBA during the busy periods (see for example, The Acoustical design of conventional open plan offices Bradley, J. S., NRCC-46274 Canadian Acoustics, v. 31, no. 2, June 2003, pp. 23-31). Thus, even though a global energy controller is used to adjust the masking sound volume, due to their instability, they still fail to ensure a precise adjustment.
U.S. Pat. No. 4,438,526 to Thomalla, issued in 1984, discloses an automatic volume and frequency-controlled masking system. In this system, a masking sound is adjusted during emission thereof according to the noise measured by microphones in the room, to obtain a constant level and a target spectrum shape of the total noise (activity noise and masking sound, together) (see page 2, line 10). The technique used to obtain a constant noise level and a target spectrum shape in the room is based on a filtering operation done on the signals coming from the microphones. The output signals of the filters are used to determine the denominator of a divided circuit. When the total noise (noise due to the activity and sound emitted by the sound-masking system, together) in the room increases, the sound emitted by the masking sound speaker is reduced in order to obtain a constant total noise level and the target spectrum shape in the room. In this system, the masking sound is thus reduced when the noise levels due to the human activity in the room increase. The objective of this system is thus very different from that of an automatic volume controller for which the objective is to increase the masking sound when the distracting sound in the room increases. Note that the system described by Thomalla does not have the instability behavior of a standard global energy controller since the masking sound is reduced when the global noise in the room increases.
As of today, a stable controller for an automatic adjustment of the masking sound volume still remains a technical challenge. Canadian patent Application CA 2, 122, 164 by A. Singmin teaches monitoring the ambient background noise to automatically adjust the volume of a white noise stimulator (generator), thereby eliminating the need to continually make a manual adjustment of the masking sound. However, this document fails to explain how the system operates and how it overcomes the instability problem.
More recently, R. Goubran and R. Botos teach an adaptive sound-masking system (US 2003/0103632 A1), based on increasing the noise level and frequency content of the masking sound according to the ambient noise. More precisely, an adaptive sound-masking system divides the sound to be masked into time-blocks and estimates the frequency spectrum and the global level, and continuously generates a white noise with a matching spectrum and global level to mask the undesired sound. In this system, the scaling factor of the amplifier is based on the standard energy controller. To avoid instability problem, the microphone is located in a first region and the speaker is located in a second region, which may limit the application of this system since the masking speaker can not be in the same region (i.e. room) where undesired sound is measured. Moreover, the system described is based on a fast adaptation rate (every 50 ms) of the masking level according to the disturbance noise (speech, phone).
Therefore, there is still a need in the art for a sound automatic volume adjustment method and system.